1. Field of the Invention
The present invention relates to a structure capable of being used for generation or detection of electromagnetic radiation, an optical semiconductor device, and a fabrication method of the structure, and particularly to techniques for generation or detection of high-frequency electromagnetic radiation such as terahertz (THz) electromagnetic radiation or wave. In this specification, the terahertz (THz) electromagnetic radiation (terahertz (THz) radiation or wave) is used for radiation in a frequency range between about 30 GHz and about 30 THz.
2. Description of the Related Background Art
A photoconductive antenna (a photoconductive device) using an optical switching device has been frequently employed for generation of high-frequency electromagnetic radiation such as THz radiation. A device for generating or detecting the THz radiation using a photoconductive element is, for example, a device which includes a photoconductive portion and two conductive portions formed on a predetermined face of the photoconductive portion in a mutually-separated manner, and in which at least portions of the two conductive portions are spaced from each other with a predetermined gap therebetween along the above-mentioned predetermined face.
The THz radiation is generated using the above device in the following manner. Even when a bias voltage is applied between the two conductive portions, almost no current flows since a resistance between the two conductive portions (a gap portion) is normally very large. Upon irradiation of the gap portion with exciting pulse light such as femtosecond pulsed laser light, free carriers are generated at the gap portion. At that moment, the resistance of the gap portion lowers, and a current flows between the conductive portions. It is accordingly possible to generate THz radiation whose electric-field amplitude is proportional to a value obtained by the time derivative of the above pulse current (a photocurrent is defined by a current, including this pulse current, generated by exciting light). At this moment, the conductive portions act as an antenna, and electromagnetic radiation with a frequency corresponding to the shape of the conductive portions is emitted toward the outside of the photoconductive device.
Antennas including dipole type, bow-tie type, strip-line type, and the like can be used for such an antenna structure. Here, generation of the THz electromagnetic radiation is guaranteed by the material, structure, and the like of the photoconductive portion, and a specific frequency of the electromagnetic radiation in a range of the THz frequency is determined by the profile of the exciting pulse light, the shape of the conductive portions and the like.
The magnitude of the bias voltage is about 20 V in the case of a photoconductive device with a conductive portion having a so-called dipole antenna structure with a gap of about 5 μm between the conductive portions. In the case of a large-aperture photoconductive device with a gap of about several centimeters between the conductive portions, the bias voltage sometimes amounts to several tens kV. In both cases, the electric field between the conductive portions is very strong, and the intensity of THz radiation to be generated increases as the electric field becomes large.
In this specification, the light-terahertz (THz) radiation (or electromagnetic radiation) converting efficiency is defined by a value obtained by dividing energy of THz radiation generated from the photoconductive device upon application of a bias voltage of 1 V between conductive portions by energy of exciting light incident on the photoconductive device.
A method of detecting the THz radiation using a photoconductive device is carried out in the following manner. A gap portion (a photoconductive portion) is irradiated with exciting pulse light, such as femtosecond pulsed laser light, to generate free carriers at the gap portion. At the same time, THz radiation is caused to impinge on the gap portion. The free carriers generated at the gap portion by the exciting pulse light are accelerated by the electric field caused by the THz radiation. Here, the THz radiation can be detected by detection of a current flowing between the photoconductive portions.
In this specification, the terahertz (THz) radiation (or electromagnetic radiation) detecting sensitivity (A/W) is defined by a value obtained by dividing a current (A) flowing between photoconductive portions by energy of the input THz radiation when the THz radiation is detected by the irradiation of the photoconductive portion with exciting light of 1 mW.
It is required for a photoconductive device having a high light-THz radiation converting efficiency that the mobility of free carriers generated at the photoconductive portion by the exciting light should be large. The reason therefor is that a value of the time derivative of a photocurrent increases as the mobility of the free carriers increases.
It is similarly required for a photoconductive device having a high THz radiation detecting sensitivity that the mobility of free carriers generated at the photoconductive portion by the exciting light should be large. This is because the free carriers can be readily accelerated by the THz radiation and a large current is created if the mobility of free carriers is large.
In a conventional example of a fabrication method of a photoconductive device, a GaAs film 52 is epitaxially grown on a semi-insulating GaAs substrate 51 under a condition in which the semi-insulating GaAs substrate 51 is maintained at a temperature below about 300° C., as illustrated in FIGS. 11A and 11B. This GaAs film 52 contains an excessive amount of arsenic precipitates, and point defects 56 are formed thereby. The point defect 56 acts as a recombination center for free carrier (see FIG. 11A). Accordingly, the carrier lifetime of the free carrier is short, and the mobility of carriers is likely to decrease (see A. C. Warren, et. al., “Arsenic precipitates and the semi-insulating properties of GaAs buffer layers grown by low-temperature molecular beam epitaxy”, Appl. Phys. Lett. 57, p. 1331, 1990).
After the epitaxial growth, the semi-insulating GaAs substrate 51 is heated at about 600° C. in an arsenic ambience using the same apparatus. The thus-fabricated GaAs film is called an LT(low-temperature-grown)-GaAs film 52a (see FIG. 11B). It is known that the LT-GaAs film has a longer carrier lifetime (i.e., a larger mobility of carriers), and a higher resistivity than the GaAs film of FIG. 1A. In other words, in the LT-GaAs film, an arsenic cluster 57 due to cohesion of the excessive arsenic precipitates is formed. In a region outside the arsenic clusters 57, there is formed a stoichiometric GaAs crystal. Accordingly, the mobility of the free carriers increases, and impurity levels for supplying free carriers decrease, leading to an enhancement of the resistivity (see FIG. 11B and A. C. Warren, et. al., “Subpicosecond, freely propagating electromagnetic pulse generation and detection using GaAs:As epilayers”, Appl. Phys. Lett., vol. 58, p. 1512, 1990).
With reference to FIGS. 9A to 9D, a description will be given for a conventional example of a fabrication method of a photoconductive device disclosed in M. Tani, et. al., “Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs”, Appl. Opt. vol. 36, p. 7853, 1997. Using a molecular-beam epitaxy, a GaAs film 52 with a thickness of 1.5 μm is grown on a (100) face of a semi-insulating GaAs substrate 51 with a thickness of 500 μm (see FIG. 9A) under a condition in which the semi-insulating GaAs substrate 51 is maintained at 250° C., as illustrated in FIG. 9B. After the epitaxial growth of the unannealed GaAs film 52, the temperature of the semi-insulating GaAs substrate 51 is raised up to 600° C. above the temperature at the time of the epitaxial growth, and the GaAs film 52 is heated for about five (5) minutes using the same apparatus 53, as illustrated in FIG. 9C. After that, conductive portions 55a and 55b with a layered structure of a first titanium thin film and a second gold thin film are formed on an annealed LT-GaAs film 52a using photolithography, as illustrated in FIG. 9D.
FIG. 10 schematically illustrates an example of a photoconductive device fabricated by the conventional fabrication method. The example illustrated in FIG. 10 has a bow-tie antenna structure with conductive portions 55a and 55b in which two symmetrical trapezoids (each being formed by truncating a 5-μm portion of a right-angled apex in a right-angled isosceles triangle with a base of 800 μm) are opposed to each other with a gap g of 5 μm therebetween.
When the photoconductive device fabricated by the above-discussed conventional fabrication method is used, an output of THz electromagnetic radiation of about 2 μW can be obtained by guiding exciting light of 12 mW and applying a bias voltage of 30 V to the photoconductive device. Further, when THz radiation is detected with such a photoconductive device, a current of about 1 nA is caused to flow by guiding exciting light of 4 mW and THz electromagnetic radiation of 1 μW to the photoconductive device.
With the THz radiation, practical applications, such as imaging and sensing, are expected, and industrialization thereof is greatly anticipated. A device with a high light-electromagnetic radiation converting efficiency is indispensable for industrial application of the THz radiation. However, the light-electromagnetic radiation converting efficiency of the above-discussed conventional photoconductive device cannot be said to be high enough.